The present invention relates to dietary fiber which can be obtained by extracting bran with an aqueous solvent at a high temperature and differs from a conventional extract of bran with an aqueous solvent in chemical composition and physiological activity; its production; and a physiologically active composition which contains the dietary fiber as an active ingredient.
It has been known that some dietary fibers can be obtained through extraction of bran with an aqueous solvent, but a low yield of the fibers obtainable by this conventional extraction method and a low physiological activity of the fibers have been problems.
For instance, Japanese Patent Application Laid-Open (KOKAI) No. 63-165,325 (1988) discloses an extraction of degreased by-products, obtained in a polishing process of grain, such as rice bran, wheat bran or corn husk, with an aqueous solution of sodium hydroxide at room temperature. By the extraction, only 4 g of hemicellulose (B) was obtained from 100 g of rice bran. Accordingly, the yield is extremely low and further, because of the extraction with an aqueous solution of NaOH, troublesome steps, such as neutralization and desalting, are necessary to purify the extract.
Japanese Patent Application Laid-Open (KOKAI), No. 64-62,303 (1989) discloses a method to extract husks of grain or beans or a cellulose obtained from the husks with hot water at a temperature of 130.degree. to 160.degree. C. and to purify the extracts, but only a hemicellulose can be obtained by the method. This reference also discloses that when a husk of grain such as bran is extracted at a temperature not lower than 160.degree. C., decomposition of the extract increases accompanied with darkening of its color and low yield of useful substance is induced. Further, a substance extracted at a temperature up to 160.degree. C. gives some deposition of hemicellulose during the purification step.
The present inventors have continuously studied to obtain a useful and physiologically active substance from natural sources. For instance, they have obtained by an extraction of Basidiomycetes with an aqueous solvent an anti-tumor substance having high molecular weight. [refer to Japanese Patent Publication, 56-28,152 (1981)]. Based on these experiences, they obtained practically indigestible dietary fibers, which is a recent topic as a so-called functional foodstuff, in a high yield. As a result, they have found that a dietary fiber is obtained in a high yield if bran is extracted by an aqueous solvent at a temperature as high as 170.degree. to 220.degree. C., contrary to common knowledge to those skilled in the art.
Further surprisingly enough, they found that in addition to the dietary fiber's physiological activity, such as promotion of digestion and absorption of nutrients and excretion of waste and proliferation of Lactobacillus bifidus within the large intestine, this unique fiber possesses very useful physiological activities, such as anti-tumor, immunological and cholesterol metabolism improving activities.
Based on these findings they have achieved the present invention.